1. Field of the Invention
The present invention is related to processes involving the removal of oxygen from fluid streams and to apparatuses used for this purpose.
2. Description of the Prior Art
Selective removal of a gas from a fluid stream is a common problem in many areas of chemistry. Examples include separating aliphatically-unsaturated hydrocarbons from mixtures containing the same (e.g. in the preparation of ethylene), recovering helium from natural gas, separating hydrogen from a petroleum cracking product, and the like. Among the methods developed for gas separation are those involving transport of gases through a membrane that retards the passage of some or all of the remaining fluid components. Examples of these processes can be found in the following U.S. Patents: recovery of helium from natural gas, U.S. Pat. No. 3,246,449; recovery of hydrogen from cracked petroleum, U.S. Pat. No. 3,246,450; removal of gas bubbles from analyte streams, U.S. Pat. No. 3,463,615; removing carbon dioxide from blood, U.S. Pat. No. 3,651,616; removing gases from liquid streams, U.S. Pat. No. 3,751,879; and separating olefins from other hydrocarbons, U.S. Pat. No. 4,239,506. Various apparatuses for carrying out these separations are disclosed in these patents and in U.S. Pat. No. 4,336,138, which discloses a permeation separation apparatus.
Typical of these disclosures is U.S. Pat. No. 3,751,879 which indicates that gases which pass through the membrane are removed by a vacuum pump, by venting to the atmosphere, by collection in an evacuated and sealed chamber, or by physical absorption or adsorption. Thus two general methods of gas removal have taken place in the prior arr: first, removal of gas from the vicinity of the membrane either by a vacuum pump or by passive diffusion into the atmosphere, and second, collection of the gas near the membrane in a vacuum chamber or physical absorbant. Both these methods suffer from disadvantages when applied to oxygen removal from fluid streams, the first requiring an expensive vacuum pump (since passive diffusion into oxygen-containing air is clearly inappropriate) and the second having a limited capacity and being difficult to monitor for loss of absorbing ability.
One area in which oxygen removal from fluid streams is very important is the field of automated luminescence measurement of analytes. Many organic compounds fluoresce or phosphoresce, and these properties are widely used for analysis. Molecules are generally excited by the absorption of ultraviolet radiation to a higher electronic state to produce measurable luminescent emission. Excited molecules rapidly lose excess energy by a variety of nonradiative de-excitation steps to the lowest excited singlet or triplet state, at which point the molecule can return to the ground state by emission of a photon. Various nonradiative de-excitation processes compete with and often greatly reduce the measurable luminescence. Of these processes, quenching has the most pronounced effects. Quenching is defined as any proces that results in a decrease in the true fluorescence or phosphorescence efficiency of a molecule. Quenching processes divert the absorbed energy of a molecule into channels other than fluorescence or phosphorescence.
The presence of molecular oxygen contributes significantly to quenching because most organic molecules in an excited state will nonradiatively deactivate after one or two collisions with molecular oxygen. Quenching is often a serious problem for phosphorescence since the longer lifetimes of the excited state allow more opportunities for collisions to occur. The effect of oxygen quenching on fluorescence is pronounced for solutions of many polynuclear aromatic compounds, but the fluorescence of virtually all organic compounds is quenched, at least slightly, by oxygen. Thus, the presence of oxygen decreases the luminescence efficiency of a sample.
Several methods of deoxygenation are currently used for preparation of fluorescent samples. These methods include nitrogen purging, freeze-thaw techniques, and preparations of samples within a vacuum. However, these methods have varying degrees of effectiveness. Furthermore, they are time-consuming and rather tedious. Thus, sample deoxygenation is usually not carried out for routine fluorometric testing despite the obvious advantages relating to fluorescence efficiency which could be obtained by deoxygenarion. Accordingly, a routine and easily carried out process for the removal of oxygen from a sample being measured in a fluorescence or phosphorescence spectrophotometer is greatly needed as is a method for removing oxygen from fluid streams in general.